.TH std::experimental::ranges::common_reference 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::experimental::ranges::common_reference \- std::experimental::ranges::common_reference

.SH Synopsis
   Defined in header <experimental/ranges/type_traits>
   template< class... T >                               (ranges TS)
   struct common_reference;

   Determines the common reference type of the types T..., that is, the type to which
   all the types in T... can be converted or bound. If such a type exists (as
   determined according to the rules below), the member type names that type.
   Otherwise, there is no member type. The behavior is undefined if any of the types in
   T... is an incomplete type other than (possibly cv-qualified) void.

   When given reference types, common_reference attempts to find a reference type to
   which the supplied reference types can all be bound, but may return a non-reference
   type if it cannot find such a reference type.

     * If sizeof...(T) is zero, there is no member type.
     * If sizeof...(T) is one (i.e., T... contains only one type T0), the member type
       names the same type as T0.
     * If sizeof...(T) is two (i.e., T... contains two types T1 and T2):
          * If T1 and T2 are both reference types, and the simple common reference type
            S of T1 and T2 (as defined below) exists, then the member type type names
            S;
          * Otherwise, if basic_common_reference<T1R, T2R, T1Q, T2Q>::type exists,
            where TiR is std::remove_cv_t<std::remove_reference_t<Ti>> and TiQ is an
            alias template such that TiQ<TiR> is Ti, then the member type type names
            that type;
          * Otherwise, if decltype(false? val<T1>() : val<T2>()), where val is a
            function template template<class T> T val();, denotes a valid type, then
            the member type type names that type;
          * Otherwise, if ranges::common_type_t<T1, T2> is a valid type, then the
            member type type names that type;
          * Otherwise, there is no member type.
     * If sizeof...(T) is greater than two (i.e., T... consists of the types T1, T2,
       R...), then if ranges::common_reference_t<T1, T2> exists, the member type
       denotes ranges::common_reference_t<ranges::common_reference_t<T1, T2>, R...> if
       such a type exists. In all other cases, there is no member type.

   The simple common reference type of two reference types T1 and T2 is defined as
   follows:

     * If T1 is cv1 X & and T2 is cv2 Y & (i.e., both are lvalue reference types):
       their simple common reference type is decltype(false? std::declval<cv12 X &>() :
       std::declval<cv12 Y &>()), where cv12 is the union of cv1 and cv2, if that type
       exists and is a reference type.
     * If T1 and T2 are both rvalue reference types: if the simple common reference
       type of T1 & and T2 & (determined according to the previous bullet) exists, then
       let C denote that type's corresponding rvalue reference type. If
       std::is_convertible<T1, C>::value and std::is_convertible<T2, C>::value are both
       true, then the simple common reference type of T1 and T2 is C.
     * Otherwise, one of the two types must be an lvalue reference type A & and the
       other must be an rvalue reference type B && (A and B might be cv-qualified). Let
       D denote the simple common reference type of A & and B const &, if any. If D
       exists and std::is_convertible<B &&, D>::value is true, then the simple common
       reference type is D.
     * Otherwise, there's no simple common reference type.

.SH Member types

   Name Definition
   type the common reference type for all T...

.SH Helper types

   template< class... T >
   using common_reference_t = typename common_reference<T...>::type;
   template< class T, class U, template<class> class TQual, template<class> class
   UQual >
   struct basic_common_reference {};

   The class template basic_common_reference is a customization point that allows users
   to influence the result of common_reference for user-defined types (typically proxy
   references). The primary template is empty.

.SH Specializations

   A program may specialize basic_common_reference<T, U, TQual, UQual> on the first two
   parameters T and U if std::is_same<T, std::decay_t<T>> and std::is_same<U,
   std::decay_t<U>> are both true and at least one of them depends on a program-defined
   type.

   If such a specialization has a member named type, it must be a public and
   unambiguous member type that names a type to which both TQual<T> and UQual<U> are
   convertible. Additionally, ranges::basic_common_reference<T, U, TQual, UQual>::type
   and ranges::basic_common_reference<U, T, UQual, TQual>::type must denote the same
   type.

   A program may not specialize basic_common_reference on the third or fourth
   parameters, nor may it specialize common_reference itself. A program that adds
   specializations in violation of these rules has undefined behavior.

.SH Notes

    This section is incomplete

.SH Example

    This section is incomplete
    Reason: no example

.SH See also

   common_type     determines the common type of a group of types
   \fI(C++11)\fP         \fI(class template)\fP
   common_type     determine the common type of a set of types
                   \fI(class template)\fP
   CommonReference specifies that two types share a common reference type
                   (concept)

.SH Categories:
     * Todo without reason
     * Todo no example
